Clear and undo do not work after loading a saved "CanvasDataUrl"

[cca-developer]

This is a follow-up to issue 29. It appears that any time an image is loaded into the canvas from the base64 encoding, the Clear, Undo, (and presumably Redo) buttons no longer work. Obviously it should not clear any old work, but when the canvas with the image is loaded, then a new line or shape is drawn onto the canvas, pressing Clear or Undo should remove that new line or shape. Currently, nothing happens (I don't see any errors, but the last action is not removed).